The present invention is concerned with the field of exhaust gas recirculation in diesel engines, and, more particularly, is concerned with a system for control of intake air flow and exhaust gas recirculation flow for a diesel engine, in which the percentage of exhaust gas recirculation provided, in accordance with the operating condition of the engine, is made more nearly directly proportional to the amount of physical movement of a valve element in the system.
A diesel engine is usually operated in a so-called "excess air" mode, in which more air is sucked into the combustion chambers of the engine than is actually required for combustion of the fuel injected thereinto. Thus, according to this, exhaust gas recirculation in a diesel engine basically consists of replacing at least a part of this surplus air by recirculated exhaust gases, in order to improve the quality of the exhaust gases from the engine, and reduce the amount of harmful pollutants therein. Thus, an amount of exhaust gas recirculation should be performed which depends on the amount of surplus air inhaled during the current operating conditions of the engine.
Generally, the lower is the load on the diesel engine, the greater is the percentage of excess air inhaled; in other words, the higher the load on the engine, the less, proportionally, of the intake air is excess air. Also, the amount of excess air introduced into the combustion chambers of the engine decreases as the load on the engine increases. Therefore, in order to carry out exhaust gas recirculation with exhaust gas recirculation amount varying in response to the excess air amount, it is necessary to control the exhaust gas recirculation amount in such a way that the exhaust gas recirculation ratio (which is the amount of recirculated exhaust gases divided by the amount of recirculated exhaust gases plus the amount of inlet air) should decrease in response to an increase in engine load, and vice versa.
In practice, in order to control exhaust gas recirculation percentage, it is known, and practiced, to control the effective cross section of the exhaust gas recirculation passage, which leads a part of the exhaust gases from the exhaust system of the engine into the air intake passage thereof, by an exhaust gas recirculation flow control valve. The amount of flow of exhaust gas through said exhaust gas recirculation passage should be controlled depending on the amount of intake air flow, and, from the point of view of the control system, it is desirable that the percentage of exhaust gas recirculation should vary in approximately direct proportion to the degree of opening of the said exhaust gas recirculation flow control valve. However, in a diesel engine, the vacuum present in the intake passage of the engine, (i.e., the inlet manifold vacuum), is rather low, compared to that in a gasoline engine, especially when the load on the engine is in the low range. Thereby, when the diesel engine is operating in the low load condition, and a large amount of exhaust gas recirculation is required, although the exhaust gas recirculation flow control valve is opened wide, and the effective cross section of the exhaust gas recirculation passage is therefore large, nevertheless, because the inlet manifold suction is not high, the amount of exhaust gas recirculated through the exhaust gas recirculation passage is not increased sufficiently. For this reason, the percentage of exhaust gas recirculation does not alter even approximately in direct proportion to the amount of opening of the exhaust gas recirculation flow control valve, as would be desirable from the point of view of the control system. Thus, when the load on the diesel engine is low, it is difficult to obtain the necessary amount of exhaust gas recirculation.